Put on to perform before closing down (3)
I believe the answer is:
don
'put on' is the definition.
(as in donning or putting on an item of clothing)
'perform before closing down' is the wordplay.
'perform' becomes 'do' (both can mean complete an action).
'before' means one lot of letters go next to another.
'closing down' becomes 'n' (I can't justify this - if you can you should believe this answer much more).
'do'+'n'='DON'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for don that I've seen before include "Put on or assume" , "Spanish male title" , "Put on; lecturer" , "Member of university" , "put on > one of several rivers" .)